1. Evidence for Single Top Quark Production at CDF Bernd Stelzer University of California, Los Angeles HEP Seminar, University of Pennsylvania September, 18th 2007

2. 2 Outline 1.Introduction to Top Quarks 2.Motivation for Single Top Search 3.The Experimental Challenge 4.Analysis Techniques Likelihood Function Discriminant (1.51fb -1 ) Matrix Element Analysis (1.51fb -1 ) 5.Measurement of |V tb | 6.More Tevatron Results 7.Summary / Conclusions / Outlook

3. 3 The Tevatron Collider Tevatron is worlds highest energy Collider (until 2008) Proton Anti-proton Collisions at E CM =1.96 TeV

4. 4 Top Production at the Tevatron Once every 10,000,000,000 inelastic collision..

5. 5 Top Production at the Tevatron At the Tevatron, top quarks are primarily produced in pairs via the strong interaction: Single top quark production is also predicted by the Standard Model through the electroweak interaction: (  st ~ ½  tt )  NLO = 6.7±0.8 pb m t =175GeV/c 2 s-channel  NLO = 0.88±0.07 pb t-channel  NLO = 1.98±0.21 pb Discovered 1995! Cross-sections at m t =175GeV/c 2, B.W. Harris et al., Phys.Rev. D70 (2004) 114012, Z. Sullivan hep-ph/0408049

6. 6 Top Quark in the Standard Model >10 orders of magnitude! Top Quark is heaviest particle to date –m t =170.9  1.8 GeV/c 2 March 2007 –Close to the scale of electroweak symmetry breaking –Special role in the Standard Model? Top Quark decays within ~10 -24 s -No time to hadronize -We can study a ‘bare quark’

7. 7 Why measure Single Top Production ? V tb Direct measurements Ratio from Bs oscillations Not precisely measured s-channel t-channel Ceccucci, Ligeti, Sakai PDG Review 2006 Precision EW rules out “simple” fourth generation extensions, but see J. Alwall et. al., “Is |V tb |~1?” Eur. Phys. J. C49 791-801 (2007). Source of single ~100% polarized top quarks: – Short lifetime, information passed to decay products – Test V-A structure of W-t-b vertex Allows direct Measurement of CKM- Matrix Element V tb : –  single top ~|V tb | 2 – indirect determinations of V tb enforce 3x3 unitarity

8. 8 Sensitivity to New Physics and Benchmark for WH Single top rate can be altered due to the presence of New Physics: - t-channel signature: Flavor changing neutral currents (t-Z/γ/g-c couplings) - s-channel signature: Heavy W boson, charged Higgs H +, Kaluza Klein excited W KK Tait, Yuan PRD63, 014018(2001) Z c t W,H+W,H+  s (pb) 1.25  t (pb) s-channel single top has the same final state as WH  l bb => benchmark for WH! (  WH ~ 1/10  s-channe )) CMSSM Study: Buchmuller, Cavanaugh, deRoeck, S.H., Isidori, Paradisi, Ronga, Weber, G. Weiglein’07]

9. 9 Experimental Challenge

10. 10 Top Pair Production with decay Into Lepton + 4 Jets final state are very striking signatures! Jet4 Jet3 Event Signatures Jet1 Jet2 Electron MET Single top Production with decay Into Lepton + 2 Jets final state Is less distinct!

11. 11   = 1.0  = 2.8  = 2.0 CDF II Detector (Cartoon) ■ Silicon tracking detectors ■ Central drift chambers (COT) ■ Solenoid Coil ■ EM calorimeter ■ Hadronic calorimeter ■ Muon scintillator counters ■ Muon drift chambers ■ Steel shielding Single top analysis needs full detector! Thanks to great work of detector experts and shift crew!

12. 12 CDF II Detector Silicon detector Central muon Central calorimeters Endplug calorimeters Drift chamber tracker

13. 13 Data Collected at CDF This analysis uses 1.51 fb -1 (All detector components ON) CDF is getting faster, too! 6 weeks turnaround time to calibrate, validate and process raw data Tevatron people are doing a fantastic job! 3fb -1 party coming up! Design goal Delivered : 3.0 fb -1 Collected : 2.7 fb -1

14. 14 Single Top Selection Event Selection: 1 Lepton, E T >20 GeV, |  e(  ) |< 2.0 (1.0) Missing E T, (MET) > 25 GeV 2 Jets, E T > 20 GeV, |  |< 2.8 Veto Fake W, Z, Dileptons, Conversions, Cosmics At least one b-tagged jet, (displaced secondary vertex tag) CDF W+2jet Candidate Event: Close-up View of Layer 00 Silicon Detector Jet2 Jet1 Electron 12mm Number of Events / 1.51 fb -1 Single TopBackground S/B W(l ) + 2 jets 13628300~1/210 W(l ) + 2 jets + b-tag 611042~1/17 Run: 205964, Event: 337705 Electron E T = 39.6 GeV, Missing E T = 37.1 GeV Jet 1: E T = 62.8 GeV, L xy = 2.9mm Jet 2: E T = 42.7 GeV, L xy = 3.9mm

15. 15 B-quark Tagging and Jet Flavor Separation Separate tagged b-jets from charm/light jets using a Neural Network trained with tracking information –L xy, vertex mass, track multiplicity, impact parameter, semilepton decay information, etc... Used in all single top analyses Neural Network Jet-Flavor Separator NN Output Charm tagging rate ~10% Mistag rate ~ 0.5% Exploit long lifetime of B hadrons (c  ~450  m)+boost L xy ~3mmB hadrons travel L xy ~3mm before decay with large track multiplicity

16. 16 Mistags (W+2jets) Falsely tagged light quark or gluon jets Mistag probability parameterization obtained from inclusive jet data Background Estimate W+HF jets (Wbb/Wcc/Wc) W+jets normalization from data and heavy flavor (HF) fraction from MC Top/EWK (WW/WZ/Z → ττ, ttbar) MC normalized to theoretical cross-section Non-W (QCD) Multijet events with semileptonic b-decays or mismeasured jets Fit low MET data and extrapolate into signal region Wbb Wcc Wc non-W Z/Dib Mistags tt W+HF jets (Wbb/Wcc/Wc) W+jets normalization from data and heavy flavor (HF) fractions from ALPGEN Monte Carlo

17. 17 Non-W Estimate Build non-W model from ‘anti-electron’ selection Require at least two non-kinematic lepton ID variables to fail: EM Shower Profile  2, shower maximum matching (dX and dZ), E had /E em, Data is superposition of non-W and W+jets contribution -> Likelihood Fit Signal Region Before b-tagging:After b-tagging: Signal Region

18. 18 W + Heavy Flavor Estimate Method inherited from CDF Run I (G. Unal et. al.) Measure fraction of W+jets events with heavy flavor (b,c) in Monte Carlo Normalize fractions to W+jets events found in data Correct data for non W+jets events Heavy flavor fractions and b-tagging efficiencies from LO ALPGEN Monte Carlo Calibrate ALPGEN heavy flavor Fractions by comparing W + 1jet Data with ALPGEN jet Monte Carlo Note: Similar for W+charm background Large uncertainties from Monte Carlo estimate and heavy flavor calibration (36%) K HF =1.4 ± 0.4

19. 19 Signal and Background Event Yield CDF RunII Preliminary, L=1.51 fb Predicted Event Yield in W+2jets CDF RunII Preliminary, L=1.51 fb -1 Predicted Event Yield in W+2jets Single top swamped by background and hidden behind background uncertainty.  Makes counting experiment impossible! s-channel23.9±6.1 t-channel37.0±5.4 Single top60.9±11.5 tt85.3±17.8 Diboson40.7±4.0 Z + jets13.8±2.0 W + bottom319.6±112.3 W + charm324.2±115.8 W + light214.6±27.3 Non-W44.5±17.8 Total background1042.8±218.2 Total prediction1103.7±230.9 Observed1078

20. 20 Analysis Flow Chart Analysis Event Selection CDF Data Monte Carlo Signal/Background Monte Carlo Signal/Background Apply MC Corrections Apply MC Corrections Analysis Technique Result Template Fit to Data Template Fit to Data Discriminant Signal Background Cross Section

21. 21 Analysis Techniques

22. 22 The Likelihood Function Analysis N sig N bkg i, index input variable Bkgr Signal Unit Area tchan schan Wbb ttbar Leading Jet E T (GeV) Uses 7 (8) kinematic variables for t-channel (s-channel) Likelihood Function e.g. M(Wb) or kin. Solver  2, H T, QxEta, NN flavor separator, Madgraph Matrix Elements, M(jj) Discriminant

23. 23 Kinematic Variables BackgroundSignalBackgroundSignal Wbb ttbar Wbb ttbar tchan schan tchan schan H T =  E T (lepton,MET,Jets) Wbb ttbar tchan schan

24. 24 Analysis Techniques

25. 25 Matrix Element Approach No single ‘golden’ kinematic variable! Attempt to include all available kinematic information by using Matrix Element approach Start from Fermi’s Golden rule: Cross-sections ~ |Matrix Element| 2  Phase space  Calculate an event-by-event probability (based on fully differential cross-section calculation) for signal and background hypothesis

26. 26 Matrix Element Method Parton distribution function (CTEQ5) Leading Order matrix element (MadEvent) W(E jet,E part ) is the probability of measuring a jet energy E jet when E part was produced Integration over part of the phase space Φ 4 Event probability for signal and background hypothesis: Input only lepton and 2 jets 4-vectors! c

27. 27 Transfer Functions E parton E jet Full simulation vs parton energy: E parto n E jet Double Gaussian parameterization: where:  E = (E parton –E jet ) Double Gaussian parameterization:

28. 28 Event Probability Discriminant (EPD) ;b = Neural Network b-tagger output We compute probabilities for signal and background hypothesis per event  Use full kinematic correlation between signal and background events Define ratio of probabilities as event probability discriminant (EPD): SignalBackground

29. 29 Event Probabilty Discriminant S/B~1/1 In most sensitive bin! S/B~1/17 over full range Likelihood fit will pin down background in low score region

30. 30 Cross-Checks

31. 31 Cross-Checks in Data Control Samples Validate method in various data control samples W+2 jets data (veto b-jets, selection orthogonal to the candidate sample) Similar kinematics, with very little contribution from top (<0.5%) pxpx pypy pzpz E Second Leading Jet Leading Leading Jet Lepton (Electron/Muon)

32. 32 Cross-Checks in Data Control Samples CDF Run II Preliminary b-tagged dilepton + 2 jets sample Purity: 99% ttbar Discard lepton with lower p T b-tagged lepton + 4 jets sample Purity: 85% ttbar Discard 2jets with lowest p T

33. 33 Monte Carlo Modeling Checks

34. 34 Template Fit to the data

35. 35 Binned Likelihood Fit Binned Likelihood Function: Expected mean in bin k: ®All sources of systematic uncertainty included as nuisance parameters ®Correlation between Shape/Normalization uncertainty considered (δ i ) β j = σ j /σ SM parameter single top (j=1) W+bottom (j=2) W+charm (j=3) Mistags (j=4) ttbar (j=5) k = Bin index i = Systematic effect δ i = Strength of effect ε ji± = ±1σ norm. shifts κ jik± = ±1σ shift in bin k

36. 36 Rate vs Shape Systematic Uncertainty Discriminant Rate systematics give fit templates freedom to move vertically only Shape systematics allow templates to ‘slide horizontally’ (bin by bin) Shape systematics Rate and Systematic uncertainties can affect rate and template shape

37. 37 Sources of Systematic Uncertainty SourceRate Uncert.Shape Uncert. W + bottom36%  W + charm36% Mistags15%  ttbar21% Non-W40%  Jet Energy Scale1..13%  Initial State Radiation3.2%  Final State Radiation5.3%  Parton Dist. Function1.4%  Monte Carlo Modeling1.6%  Efficiencies/b-tag SF5% Luminosity6% CDF RunII Preliminary, L=1.51fb -1

38. 38 Results

39. 39 Matrix Element Analysis Matrix Element analysis observes excess over background expectation Likelihood fit result for combined search:

40. 40 ME Separate Search s-channel  s =1.1 pb +1.0 −0.8 t-channel  t = 1.9 pb +1.0 −0.9 Perform separate likelihood fit for s-channel and t-channel signal Both signal templates float independently

41. 41 Likelihood Function Discriminant Likelihood Function analysis also observes excess over background expectation Observed deficit previously in 0.955 fb -1

42. 42 Likelihood Function 2D Fit

43. 43 Signal Significance

44. 44 Hypothesis Testing Calculate p-value: Faction of background- only pseudo-experiments with a test statistic value as signal like (or more) as the value observed in data Define Likelihood ratio test statistic: Systematic uncertainties included in pseudo-experiments Use median p-value as measure for the expected sensitivity Median p-value = 0.13% (3.0  ) More signal like Less signal like Observed p-value = 0.09% (3.1  ) L. Read, J. Phys. G 28, 2693 (2002) T. Junk, Nucl. Instrum. Meth. A 434, 435 (1999) 3.1  Evidence

45. 45 Hypothesis Testing Median p-value = 0.20% (2.9  ) More signal like Less signal like Observed p-value = 0.31% (2.7  )

46. 46 Signal Features

47. 47 Central Electron Candidate Charge: -1, Eta=-0.72 MET=41.85, MetPhi=-0.83 Jet1: Et=46.7 Eta=-0.61 b-tag=1 Jet2: Et=16.6 Eta=-2.91 b-tag=0 QxEta = 2.91 (t-channel signature) EPD=0.95 Single Top Candidate Event Jet1 Jet2 Lepton Run: 211883, Event: 1911511

48. 48 Single Top Signal Features EPD>0.90 EPD>0.95 Look for signal features in high score output

49. 49 QxEta Distributions in Signal Region EPD>0.9 3)4) EPD>0.95

50. 50 m(W,b) Distributions in Signal Region EPD>0.9EPD>0.95

51. 51 Unconstrained Likelihood Fit Remove all background normalization constraints and perform a five parameter likelihood fit (all template shapes float freely)  Best fit for signal almost unchanged.  Uncertainty increased by about 20%

52. 52 Direct |V tb | Measurement Using the Matrix Element cross Section PDF we measure |V tb | Assume Standard Model V-A coupling and |V tb | >> |V ts |, |V td | |V tb |= 1.02 ± 0.18 (experiment) ± 0.07 (theory) t-channel Z. Sullivan, Phys.Rev. D70 (2004) 114012 Flat prior 0 < |V tb | 2 < 1 |V tb |>0.55 at 95% C.L. s-channel

53. 53 Single Top Results from DØ

54. 54 D0 Results First direct limit on V tb : 0.68 <|V tb |< 1 @ 95% CL or |V tb | = 1.3 ± 0.2 First direct limit on V tb : 0.68 <|V tb |< 1 @ 95% CL or |V tb | = 1.3 ± 0.2 Boosted Decision Tree PRL 98 18102 (2007) Expected p-value = 1.9% (2.1  ) Observed p-value = 0.04% (3.4  ) 3.4  Evidence

55. 55 Summary of Results Expected 3.0  2.9  2.6  2.1  1.9  2.2  Observed 3.1  2.7  3.4  3.2  2.7  Summary CDF analyses more sensitive D0 observes upward fluctuation In 900 pb -1 of data Combined: 2.3  / 3.6 

56. 56 CDF Single Top History First Tevatron Run II result using 162 pb -1  single top < 17.5 pb at 95 % C.L. 2004: Simple analysis while refining Monte Carlo samples and analysis tools 2 Years 2006: Established sophisticated analyses Check robustness in data control samples 2007: Evidence for single top quark production using 1.5 fb -1 (expected and observed!) Development of powerful analysis techniques (Matrix Element, NN, Likelihood Discriminant) NN Jet-Flavor Separator to purify sample Refined background estimates and modeling Increase acceptance (forward electrons) 10x more data Phys. Rev. D71 012005

57. 57 Conclusion Evidence for electroweak single top quark production at the Tevatron established by CDF and D0 experiment! First direct measures of CKM matrix element |Vtb| Advanced analysis tools essential to establish small signals buried underneath large backgrounds Entering the era of single top physics. 4-5 sigma observation possible with >3 fb -1 of data - Perhaps CDF is lucky this time.. Separate s-channel from t-channel, measure more top properties, e.g. top polarization etc.. Exciting times! The race for first observation is on.. Important milestone along the way to the Higgs!

58. 58 Search for Heavy W Boson Limit at 95% C.L. M(W´) > 760 GeV/c 2 for M(W´) > M( ν R ) M(W´) > 790 GeV/c 2 for M(W´) < M( ν R ) W Search for heavy W boson in W + 2, 3 jets Assume Standard Model coupling strengths (Z. Sullivan, Phys. Rev. D 66, 075011, 2002) Perform fit to M Wjj distribution Previous Limits: CDF Run I: M(W R ) > 566 GeV/c 2 at 95% C.L. D0 Run II: M(W R ) > 630 GeV/c 2 at 95% C.L.

59. 59 LHC is the Future Large Hadron Collider

60. 60 LHC is the Future LHC will be a top quark factory σ tt ~ 800 pb σ t-channel ~ 243 pb (153 pb for top and 90 pb for antitop production) σ s-channel ~ 11 pb (6.6 pb for top and 4.8 pb for antitop production) σ Wt ~ 50-60 pb (negligible at the Tevatron) First precision t-channel measurement (10%) expected after 1 st year of running (10 fb -1 /year) s-channel measurement harder because of small cross section and large backgrounds (sounds familiar!) The associated Wt production is tough because of large top-pair background (W+3jets signature) Wt- production Additional single top process at the LHC! (negligible at the Tevatron)

61. 61 Backup Slides Backup